This invention relates to processes for transitioning between polymerization catalyst systems, especially catalyst systems that are incompatible with each other. More particularly, the invention relates to processes for transitioning from olefin polymerization reactions utilizing chromium-based catalyst systems to olefin polymerizations utilizing metallocene/Ziegler-Natta mixed catalyst systems and vice-versa.
During the production of olefin polymers in a commercial reactor it is often necessary to transition from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system capable of producing polymers of different chemical and/or physical attributes. Transitioning between similar catalyst systems, or compatible catalyst systems, for instance, generally takes place easily. However, where the catalyst systems are of different types or are otherwise incompatible, the process is typically complicated. For example, transitioning from a chromium-based catalyst to a metallocene catalyst, or vice versa, normally requires a long transition period. In addition, the polyolefins produced during this transition period will continuously change in properties resulting in off-grade products. If the transition from one catalyst system to another requires substantial changes in reactor conditions, the risks of encountering production problems and producing polymers having extreme properties are increased.
When a polymerization reaction catalyzed by a first catalyst is to be changed to a polymerization reaction catalyzed by a second catalyst wherein the second catalyst is incompatible with the first catalyst, several methods of performing the transition are possible. One possibility is to kill the existing polymerization reaction, empty the reactor, recharge and start again with a new catalyst. The benefit of this procedure is that the amount of remaining material in the plant from the previous run will be small. The disadvantages of this procedure are that it takes several hours to build up a desired level of solids inside the reactor and traces of material from the first polymerization reaction remain in the reactor since it is practically impossible to completely remove such material.
Another possibility is to make a running transition where the change from one type of produced polymer to another is continuous, that is without interrupting the polymerization reaction. When transitioning from a first to a second catalyst, the initial step is to discontinue the catalyst feed. The new catalyst is then introduced and, if necessary, the reactor conditions are adapted to the conditions required by the new catalyst.
U.S. Pat. No. 6,284,849 to Almquist et al. discloses a method for transitioning between a chromium-based catalyst and a metallocene. The examples therein utilize Phillips type chromium-based catalysts and bisCp metallocenes. So-called Phillips type chromium-based catalysts are chromium oxide-on-silica catalysts. The catalysts are formed by impregnating a Cr+3 species into silica, followed by fluidization of the silica matrix at ca. 400xc2x0 C.-900xc2x0 C. wherein Cr+3 is converted to Cr+6. The Phillips type chromium-based catalyst is also commonly referred to in the prior art as xe2x80x9cinorganic oxide-supported Cr+6xe2x80x9d.
It would be highly advantageous to have a process for transitioning between different or incompatible catalyst systems, without the need for halting the polymerization reaction, emptying the reactor to rid it of the original catalyst system and restarting the polymerization reaction with another catalyst system. In addition, it would be advantageous if the process for transitioning could reduce the amount of off-grade material produced during the transition process, reduce the transition time, increase the robustness and stability of the transition process and avoid the need to open the reactor to charge the seed bed.
A process of transitioning from a first polymerization reaction conducted in the presence of a mixed catalyst system to a second polymerization reaction conducted in the presence of a chrome-based catalyst system is disclosed, the polymerization reactions being conducted in one embodiment in a polymerization zone of a gas phase fluidized bed reactor which contains a fluidized bed of polymer particles by the essentially continuous passage of monomer gases through the polymerization zone, comprising:
a) discontinuing the introduction of the mixed catalyst system into the reactor;
b) maintaining polymerization conditions in the reactor and permitting polymerization to continue for a period of time to allow the components of the mixed catalyst system present in the reactor to produce additional polymer particles;
c) introducing, for example by injecting, a deactivating agent into the fluidized bed in an amount sufficient to deactivate the mixed catalyst system;
d) establishing optimal conditions within the reactor for the chrome-based catalyst system; and
e) introducing the chrome-based catalyst system into the reactor;
characterized in that the amount of polymer particles in the fluidized bed is substantially maintained in the polymerization zone during the transition steps.